Unfurling banners that read "STOP HATE" and "YOUR HATE IS KILLING PEOPLE," the protesters began chanting "black lives matter" as Trump addressed a crowd of several thousand people in an airport hangar.

The group of protesters were one of a half-dozen interruptions by protesters during Trump's rally.

Security personnel -- including event security, police officers and several former members of Trump's private security detail dressed in civilian clothes -- took at least six minutes to eject the protesters, several of whom forcefully resisted being led out of the venue.

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Trump campaign manager Corey Lewandowski also headed into the crowd in an apparent attempt to help security box in the protesters and move them toward the exit.

Trump supporters shouted at the protesters as they chanted the slogan of the Black Lives Matter movement, which is aimed at denouncing police brutality and racial discrimination.

Several protesters fell to the ground as security attempted to remove them from the venue.

The protests come after a day in which Trump doubled down on his attacks on rivals Texas Sen. Ted Cruz and Florida Sen. Marco Rubio, as well as 2012 GOP nominee Mitt Romney, who is urging Republicans to stop Trump's march to the Republican nomination.

"Millions and millions of people -- more than four years ago -- are coming out and they're voting Republican," Trump said in a subtle dig at Romney. "It's not because of Lyin' Ted and it's not because of Little Marco. It's because of Trump."

Hours before Thursday's Republican debate, Romney slammed Trump as a "phony" and a "fraud" and urged Republicans to support Trump's rivals -- laying out a path toward a brokered convention that could keep Trump from becoming the Republican Party's presidential nominee.

During three events on Friday, Trump repeatedly slammed Romney for losing to Obama and for now knocking him in the race.

"He's making a fool out of himself," Trump said earlier in the day at a rally in Cadillac, Michigan.